X-rays are widely used for medical imaging and security, but they still are dangerous. Would it be possible to replace X-rays by an imaging technology based on microwave radiation? Until now, it was not feasible because portable devices were not powerful enough. Now, two U.S. scientists have found how to make microwaves on a chip to replace X-rays for medical imaging and security. Their new method is based on a phenomenon known as nonlinear constructive interference. With this method, it is possible to generate high-power signals at frequencies of 200 GHz and higher on ordinary silicon chips. This technology might be used in a few years by physicians to detect skin cancer or by airport security personnel to find objects hidden under clothing. But read more...
First, what is linear constructive interference? You can see it on the illustration on the left. The top part shows that it "occurs when two signals in phase combine to form a new signal whose amplitude is the sum of the two sources. The bottom part of the picture shows that "in a square lattice designed to create nonlinear interference, low-power signals from the bottom and side combine many wave peaks into one with a much higher amplitude." (Credit: Ehsan Afshari and Harish Bhat) These pictures can be found on this page at Cornell University.
This method for generating high-power signals at frequencies of 200 GHz and higher on silicon chips has been developed by Ehsan Afshari, Cornell assistant professor of electrical and computer engineering, who is the rincipal investigator of Cornell's Ultra high-speed Nonlinear Integrated Circuit lab (UNIC). He worked with Harish Bhat, assistant professor of mathematics at the University of California-Merced.
Now, let's look in detail at the method proposed by the two researchers. "Afshari and Bhat propose to use a phenomenon known as nonlinear constructive interference. Linear constructive interference occurs when two signals that are in phase -- that is, with their peaks and valleys matched -- produce a new signal as large as both added together. But if the signals are traveling through an uneven medium, the waves can become distorted, some delayed, some moving ahead to produce a "nonlinear" result that combines many small waves into fewer large peaks. Afshari likens the effect to the breaking of waves on the seashore. In the open ocean, waves travel as smooth undulations. But near shore the waves encounter an uneven surface at varying depths and become distorted into breakers."
But how can you create this effect on a chip? "The researchers propose a lattice of squares made up of inductors -- the equivalent of tiny coils of wire -- with each intersection grounded through a capacitor. An electrical wave moves across the lattice by alternately filling each inductor then discharging the current into the adjacent capacitor. A capacitor temporarily stores and releases electrons, and these capacitors, made of layers of silicon and silicon dioxide, are designed to vary their storage capacity as the voltage of the signal changes, creating the equivalent of the varying depths of an ocean beach and distorting the timing of the electrical signals that pass by."
I don't know if this method will be widely used in the years to come. Nevertheless, the technology sounds promising. "According to computer simulations by Afshari and Bhat, the process can be implemented on a common complimentary metal-oxide silicon (CMOS) chip to generate signals at frequencies well above the ordinary cutoff frequencies of such chips, with at least 10 times the input power."
This research work has been accepted for future inclusion in Physical Review E (Volume 77, Number 5, May 2008) under the name "Nonlinear constructive interference in electrical lattices." Here is a link to the abstract (this is a temporary link, which will probably change when the paper is published). "We analyze constructive interference of input signals in nonlinear, discrete, two-dimensional media. The media consist of inductor-capacitor lattices with saturating, voltage-dependent capacitors. We find that nonlinearity significantly boosts the ability of such media to generate large-amplitude output signals from small-amplitude inputs. To understand this boosting, we develop a general perturbative method suitable for finding the steady-state solution of a damped NxN nonlinear lattice that is driven at a single frequency. We verify our theory using extensive numerical simulations."
Finally, here is a link to a preprint version of the -- highly technical -- paper (PDF format, 31 pages, 557 KB). If you don't have a degree in mathematics and/or in physics, please avoid it.
Sources: Bill Steele, Cornell Chronicle Online, May 29, 2008; and various websites
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